They aren't exactly a necessity. If you don't make much money, don't buy one.
I really can't see the reasoning behind that statement. What other ways are there (besides practicing on an actual drum, which is often not possible for many people) to improve your drumming technique? Maybe you should rephrase that comment as "If you don't want to ever get good at playing drums, don't buy one."
Practice pads
are worth it if you plan to improve your percussion skills. Even if you are lucky enough to own your own drum, chances are you do not get much opportunity to play it, due to potential conflict with neighbors, family, etc.
Back on topic.... Well, as Dave correctly pointed out, practice pads aren't a very in-demand product, so to compensate, the manufacturers and retailers must raise the prices. The rest probably comes from the gum rubber used to make most pads, which, I assume, is the most expensive component (in most cases.) If you want to do a little detective work, you could look up prices for bulk gum rubber sheets. Maybe even do a little DIY and put your own pad together with some help from your local hardware store, which would likely have all the necessary components in stock (I believe there are some on this forum that have done this.)